[Test] Add comprehensive tests for URL deep processing

[Test] Add comprehensive tests for URL deep processing

Unit tests (test/lua/unit/lua_url_filter.lua):
- filter_url_string basic validation (normal, long user, multiple @)
- filter_url with URL objects
- UTF-8 validation (ASCII, Cyrillic, Japanese, invalid)
- Custom filter registration and chaining
- Issue #5731 regression test (oversized user parsing)

Functional tests (test/functional/cases/001_merged/400_url_suspect.robot):
- Moved to 001_merged for shared setup/teardown
- Long user field (80 chars) - URL_USER_LONG
- Very long user field (300 chars) - URL_USER_VERY_LONG
- Numeric IP - URL_NUMERIC_IP
- Numeric IP with user - URL_NUMERIC_IP_USER
- Suspicious TLD - URL_SUSPICIOUS_TLD
- Multiple @ signs - URL_MULTIPLE_AT_SIGNS
- Normal URLs (no false positives)
- All tests verify R_SUSPICIOUS_URL backward compatibility

Test messages (test/functional/messages/):
- url_suspect_long_user.eml (80-char user)
- url_suspect_very_long_user.eml (300-char user)
- url_suspect_numeric_ip.eml
- url_suspect_numeric_ip_user.eml
- url_suspect_bad_tld.eml
- url_suspect_multiple_at.eml
- url_suspect_normal.eml

Config:
- Enable url_suspect plugin in merged-override.conf
- Add Robot Framework outputs to gitignore